Process for applying a protective transparent coating to zinc and cadmium and composition therefor



United States Patent PRBCESES Fill? APPLYENG A PROTECTIVE TRAN= PARENT CGATTNG T0 ZINC AND CADMIUM AND CGMPGSITKON THEREFGR Edgar .i. Seyb, Jr., Oak Park, Mich, assignor, by mesne assignments, to M & T Chemicals End, New York, N.Y., a corporation of Delaware No Drawing. Filed Sept. 27, 1960, Ser. No. 58,625

12 Claims. (Cl. 148-621) The present invention relates to a dip process for forming surface conversion coatings on zinc and cadmium.

Large quantities of steel are galvanized to achieve a corrosion-resistant and attractive finish. This material has a tendency to form a film of White corrosion products in storage. The actual corrosion damage caused by this filming is slight; however, it is unsightly and highly undesirable on new items presented to the public for sale. Surface plating with cadmium is also an important industrial process. The same general considerations apply for protecting cadmium and zinc surfaces. Such surface treatments should be inexpensive. It is highly desirable that the treatment be one that can be applied to the surface at a relatively high temperature. I have now discovered an aqueous bath for applying protective coatings to zinc and cadmium surfaces which meets the needs of the industry.

It is an object of this invention to provide a bath for applying protective coatings to zinc surfaces and to cadmium surfaces by immersion.

It is also an object of this invention to provide mixtures of chemicals which when dissolved in water and adjusted to the proper pH yield baths useful for immersion coating of zinc or of cadmium.

The invention also contemplates providing a process for surface coating zinc or cadmium by a simple, one-step immersion process.

I discovered that a clear protective film is applied to zinc-surfaced or to cadmium-surfaced objects, and particularly to galvanized iron, by immersing the object into an aqueous bath having concentrations between 3 and 20 g./l. (grams per liter) of chromic acid (hereinafter referred to as CrO between 0.45 and 3.2 g./l. of sulfate ion, between 0.7 and 6 g./l. of nitrate ion, and between 0.2 to 1.6 g./1. of acetate or borate. The ratio of chromic acid to sulfate must be such that approximately to 15 times as much chromic acid as sulfate is present (by weight). The solution is operative at a pH between 1.5 and 3. The preferred pH Within this range is dependent upon the time of immersion (dip time). For short dip times (1 to 10 seconds), pH values at the lower end of the range are preferred. For longer dip times (20 to 30 seconds and sometimes as long as 60 seconds) pH values at the higher end of the range are preferred. Optimum results are obtained for short dip times at a pH of about 2; at dip times of 20 to 30 seconds, at a pH of about 2.6. It is not contemplated that the dip time will exceed 60 second since longer times do not appreciably improve the corrosion resistance or enhance the appearance. Corrosion protection for a specified dip time is better at the lower portion of the pH range. Coatings obtained at the upper portion of the pH range have greater clarity. The pH is maintained by additions of nitric acid. The bath is operative from room temperature to the boiling temperature of the "ice solution, although it is preferred to operate below 76 C.

A preferred range of concentrations for the bath as well as optimum concentrations for most industrial purposes is specified in Table I.

The baths of this invention are prepared by dissolving the respective acids or acid salts or salts of the desired anions in Water. Salts are Water-soluble. The choice of source of the anions will vary with economics and other factors such as case of material handling, storage, etc. The entire bath could be prepared from liquid acids. For the average plant, it should be most economical to purchase the salts in a solid composition which when dissolved in the recommended quantity of water results in the specified bath. Caustic soda would generally be used to raise the pH when the bath is prepared from all liquid acids. For solid compositions, dependent upon the salts utilized and the amounts, it will be necessary to adjust the pH. Nitric acid is preferred for lowering the pH. Chromic acid (CrO may be added as such or in the form of such equivalent salts as sodium bichromate, potassium trichromate, potassium tetrachrornate, ammonium bichromate, zinc trichromate trihydrate, the chromate of sodium, potassium, lithium, zinc, ammonium, etc. The sulfate and nitrate ions may be added in the form of the acids or such salts as sodium sulfate, sodium hydrogen sulfate, potassium sulfate, potassium bisulfate, zinc sulfate, potassium nitrate, sodium nitrate, zinc nitrate, etc. The acetate and borate are usually added as the respective acids or salts, such as sodium acetate or diacetate, boric acid, sodium borate, etc.

Compositions useful for making up and/ or maintaining dip solutions may be broadly defined as consisting essentially of the following:

TABLE 11 Parts by Weight Broad Preferred GrO 3-20 5-10 Sulfate 0. 45-3. 2 0. 8-1. 6 Nitrate 0. 7-6 1. 5-3 Acetate 0r borate 0. 2-1. 6 (1.4-0.8

which is relative to the given composition and is not based on an assumption that the total composition will be 100 parts, or any other set amount. The total composition Will be the sum of all the components, which for the anion components thereof could vary from 4.35 to 30.8 parts. Specific examples of compositions of matter which when dissolved in water in recommended proportions yield operative and useful baths follow:

Percent Composition 1 Na CI' O ZI-I O Na SO 11.0 NHCZHSOZ NaNO 11.6 Composition 2 CrO 64.2 NaHSQ; 12.8 I-I BO 5.1 NaNO 17.9 Composition 3 (liquid concentrate) CrO 29.8 NaHSO 7.2 HO C H (glacial) 2.7 HNO 3.9 H 56 4 Composition 4 NflgCI'zOq'ZHzO NaHSO 12.5 H BO 7.0 NaNO 20.5 Composition 5- NEZCI'ZOI'I'ZHZO Na SO 12.0 NHCgHgOz NaNO 5.0 Composition 6- Na Cr O -2H O 75.0 N21 50: NaNO- 10.0 NBCgHaO 5-0 Baths made up from the noted compositions were used to coat objects as noted.

Example 1 A bath was made up with 16.4 g./l. of Composition 1. 0.8 cc./1. of 42 B. HNO acid was added to bring the pH to 2.6. With this concentration of salts, the equivalent concentration of anions was as follows:

G./l. CrO 8.0 $0.; 1.2 CgHgOz 5 N0 2 1 This bath was used at 32 C. to dip a strip of galvanized steel for 15 seconds. After rinsing, the part was clear and bright.

Example 2 A bath was prepared with 16.4 g./l. of Composition 1. 1.5 cc./l. of 42 B. were added to bring the pH to 1.7.

The bath was heated to 76 C. and a strip of clean galvanized steel was dipped in the solution for 1 second. After rinsing and drying the zinc surface was bright and clear.

4 Example 3 A bath was prepared with 28 g/l. of Composition 1. 1.4 cc./l. of 42 B. HNO were added to bring the pH This bath was warmed to 32 C. and a galvanized strip was immersed in it for 30 seconds. drying, the surface was bright but with a slight yellow tinge to the coating.

Example 4 A bath was prepared with 7.8 g./l. of Composition 2. 5.2 cc./l. of NaGH solution (400 g. of NaOH per liter) were added to bring the pH to 2.0. The concentration of anions was:

G./l. CrO 5 0 S0 0.8 Borate 0.4 N0 10 The bath Was heated to 60 C. and a strip of galvanized steel was immersed for 30 seconds. After rinsing and drying, the surface of the zinc was bright but had a slight yellow-brown coloration.

Example 5 A bath was prepared by taking 25 cc. of the liquid concentrate according to Composition 3 and mixing in sufiicient water to make a liter of solution. 11.7 cc. of NaOl-I solution (400 g./l. NaOH) was added to give a pH of 1.7. This bath then had an anion concentration as follows:

CrO 10.0 80.; 1.9 C H O .9 3 1.2 This bath was heated to 70 C. and a galvanized steel strip was immersed to 2 seconds. bright and clear.

The zinc surface was Example 6 A bath was prepared with 10 g./l. of Composition 4 and adjusted to a pH of 2.4 by the addition of 0.6 cc./l. of caustic solution (400 g./l. NaOI-I). The anion con centration was:

This bath was warmed to 25 C. and a galvanized strip was immersed for 45 seconds. slight yellow-brown coloration.

The zinc surface had a Example 7 A bath was prepared with 12.0 g./l. of Composition 5. 0.8 cc./l. of 42 B. HNO was added to obtain a pH of 2.6. The anion concentration was:

CrO 64 S0 10 021 1302 .45 N03 1.2

This bath was heated to 65 C. and a strip of galyanized steel was immersed for 15 seconds. After rinsing and drying, the zinc surface was bright and clear.

After rinsing and V Ex'ample 8 A bath was prepared with 12.0 g./l. of Composition 5. 1.9 cc./1. of 42 B. HNO was added to obtain a pH of 1.7. The anion concentration was:

CrO 6.4 $0 1.0 C2H302 .45

This bath was heated to 65 C. and a strip of galvanized steel was dipped for 2 seconds. After rinsing and drying the zinc surface was bright and clear. Another galvanized strip was immersed for 15 seconds. After rinsing and drying the zinc was bright with a yellowish cast.

Example 9 A bath was prepared with 14.4 g./l. of Composition 6. 0.7 cc./l. of 42 B. HNO was added to give a pH of 2.8. The anion concentration was:

G./l. CI'O3 so 1.0 C H O .5 N0 1.7

This bath was heated to 45 C. and a strip of galvanized zinc was immersed for 45 seconds. After water rinsing and drying, the zinc surface was bright and clear with a very slight yellow cast.

Example 10 A bath was prepared with 14.4 g./l. of Composition 6. 1.3 cc./l. of 42 Be. HNO was added to give a pH of 2.0. The anion concentration was:

G./l. CrO 7.2 $0.; 1.0 C H O .5 N0 2.3

This bath was heated to 50 C. and a strip of galvanized zinc was immersed for 3 seconds. After water rinsing and drying the Zinc surface was bright and clear.

Dip tests were also conducted with some of the above baths using cadmium or zinc plated steel strips. The numbers refer to the above examples. The same conditions were used for dipping the cadmium and zinc plates.

Several of the above test pieces were tested for prevention of zinc corrosion products in the following test procedure. The test pieces were placed over glass tumblers approximately 25% filled with distilled water. The test pieces were weighted so that a snug fit was obtained over the glass tumblers. The water was then heated until condensation appeared on the bottom sides of the test pieces. The assembly was then allowed to cool and remain at room temperature during the remainder of the test period. The control panels with untreated Zinc showed light white coloration of zinc corrosion product within minutes, a heavier readily seen White product within an hour. The zinc-coated panels treated as described in Example 7 showed no white coloration after 36 hours exposure, with only light corrosion after 60 hours exposure. The Zinc- 6 coated panels treated as described in Example 8 with a 15 second dip time showed no corrosion products after 82 hours exposure.

The coatings may be formed in continuous operations. As relatively short dip times may be utilized, it is possible to use this process in extremely high speed coating of wire and sheet. The processes and baths are useful for coating zinc and high zinc alloys as Well as cadmium and high cadmium alloys.

As many embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention includes all such modifications and variations as come within the scope of the appended claims.

I claim:

1. An aqueous chromic acid bath useful in the immersion coating of zinc and cadmium, having a pH between 1.5 and 3 and consisting essentially of between 3 g./l. and 20 g./l. of chromic acid, between 0.45 g./l. and 3.2 g./l. of sulfate ions, between 0.7 g./l. and 6 g./l. of nitrate ion, and between 0.2 g./l. and 1.6 g./l. of at least one ion selected from the group consisting of acetate and borate, the ratio of chromic acid to sulfate being between 5:1 and 15:1.

2. An aqueous chromic acid bath useful in the immersion coating of zinc and cadmium, having a pH between 2 and 3 and consisting essentially of between 5 g./l. and 10 g./l. of chromic acid, between 0.8 g./l. and 1.6 g./l. of sulfate ions, between 1.5 g./l. and 3 g./l. of nitrate ion, and between 0.4 g./l. and 0.8 g./l. of at least one ion selected from the group consisting of acetate and borate, the ratio of chromic acid to sulfate being between 5 :1 and 15:1.

3. An aqueous chromic acid bath useful in the immersion coating of zinc and cadmium, having a pH between 2 and 2.8 and consisting essentially of about 7.5 g./l. of chromic acid, of about 1.2 g./l. of sulfate ion, of about 2.2 g./l. of nitrate ion, and about 0.6 g./l. of at least one ion selected from the group consisting of acetate and borate.

4. A process for applying a protective transparent coating to zinc and cadmium comprising immersing for more than 5 seconds one of said metals in an aqueous chromic acid bath having a pH between 1.5 and 3 and consisting essentially of between 3 g./l. and 20 g./l. of chromic acid, between 0.45 g./l. and 3.2 g./l. of sulfate ion, between 0.7 g.l. and 6 g.l. of nitrate ion, and between 0.2 g./l. and 1.6 g./l. of at least one ion selected from the group consisting of acetate and borate, the ratio of chromic acid to sulfate being between 5 :1 and 15 :1.

5. A process for applying a protective transparent coating to zinc and cadmium comprising immersing for between 1 and 60 seconds one of said metals in an aqueous chromic acid bath having a pH between 2 and 3 and consisting essentially of about 7.5 g./l. of chromic acid, of about 1.2 g./l. of sulfate ion, of about 2.2 g./l. of nitrate ion, and about 0.6 g./l. of at least one ion selected from the group consisting of acetate and borate.

6. A process for applying a protective transparent coating to Zinc and cadmium comprising immersing for between 1 and 10 seconds one of said metals in an aqueous chromic acid bath having a pH between 1.5 and 2 and consisting essentially of between 5 g./l. and 10 g./l. of chromic acid, between 0.8 g./l. and 1.6 g./l. of sulfate ion, between 1.5 g./l. and 3 g./l. of nitrate ion, and between 0.4 g./l. and 0.8 g./l. of at least one ion selected from the group consisting of acetate and borate, the ratio of chromic acid to sulfate being between 5 :1 and 15 :1.

7. A process for applying a protective transparent coating to zinc and cadmium comprising immersing for between 15 and 45 seconds one of said metals in an aqueous chromic acid bath having a pH of between 2.4 and 2.8 and consisting essentially of between 5 g./l. and 10 g./l. of chromic acid, between 0.8 g./l. and 1.6 g./l. of

7 sulfate ion, between 1.5 g./l. and 3 g./l. of nitrate ion, and between 0.4 g./l. and 0.8 g./l. of at least one ion selected from the group consisting of acetate and borate, the ratio of chromic acid to sulfate being between 5:1 and 15:1.

8. A process for applying a protective transparent coating to zinc and cadmium comprising immersing between 20 and 30 seconds one of said metals in an aqueous chromic acid bath having a pH of about 2.6 and consisting essentially of between 5 g./l. and 10 g./l. of chromic acid, between 0.8 g./l. and 1.6 g./1. of sulfate ion, between 1.5 g./l. and 3 g./l. of nitrate ion, and between 0.4 g./l. and 0.8 g./l. of at least one ion selected from the group consisting of acetate and borate, the ratio of chromic acid to sulfate being between 5:1 and 15 :1.

9. A composition of matter for making up and for maintaining aqueous chromic acid baths used to impart a protective transparent coating to zinc and cadmium articles, consisting essentially of a mixture of compounds selected from the Water soluble salts of chromic acid, sulfuric acid, nitric acid, and acetic acid, in which the anion portion of said salts are present in the following amounts:

Parts by Weight CrO 3-20 Sulfate OAS-3.2 Nitrate 0.7-6 Acetate 0.2-1.6

ticles, consisting essentially of between 0.2 part by Weight and 1.6 parts by weight of boric acid and a mixture of compounds selected from the water soluble salts of chromic acid, sulfuric acid, and nitric acid, in which the anion portion of said salts are present in the following amounts:

Parts by weight CrO 3-20 Sulfate 0.45-3.2 Nitrate 0.7-6

-CrO 5-10 Sulfate 0.8-1.6 Nitrate 1.5-3 Acetate 0.4-0.8

the ratio of CrO to sulfate being between 5:1 and 15 :1. 12. A composition of matter for making up and for maintaining aqueous chromic acid baths used to impart a protective transparent coating to zinc and cadmium atricles, consisting essentially of a mixture of compounds selected from the water soluble salts of chromic acid, sulfuric acid, nitric acid, and boric acid, in which the anion portion of said salts are present in the following amounts:

Parts by weight CrO 5-10 Sulfate 0.8-1.6

Nitrate 1.5-3

Borate 0.4-0.8

the ratio of CrO to sulfate being between 5:1 and 15 :1.-

References Cited in the file of this patent UNITED STATES PATENTS 2,483,510 Stareck Oct. 4, 1949 2,628,925 Ostrander Feb. 17, 1953 2,824,031 Stareck Feb. 18, 1958 2,904,414 Ostrander et al Sept. 15, 1959 

1. AN AQUEOUS CHROMIC CID BATH USEFUL IN THE IMMERSION COATING OF ZINC AND CADMIUM, HAVING A PH BETWEEN 1.5 AND 3 AND CONSISTING ESSENTIALLY OF BETWEEN 3 G./L. AND 20 G./L. OF CHROMIC ACID, BETWEEN 0.45 G./L. AND 3.2 G./L. OF SULFATE IONS, BETWEEN 0.7 G./L. AND 6 G./L. OF NITRATE ION, AND BETWEEN 0.2 G./L. AND 1.6 G./L. OF AT LEAST ONE ION SELECTED FROM THE GROUP CONSISTING OF ACETATE AND BORATE, THE RATIO OF CHROMIC ACID TO SULFATE BEING BETWEEN 5:1 AND 15:1. 